Many volatile compounds identified in complex biological matrixes are chiral molecules. The chirality evaluation is of great importance in drugs and pheromone chemistry, but chirality also plays a very important role in flavour and fragrance chemistry. Analyses of these products are performed to determine their composition, to control their quality and to identify adulterations or product counterfeiting. In this respect, proper characterizations of the enantiomeric composition can provide information on the genuineness, geographic origin, biogenesis and quality of the product.
3-hydroxy-3-methyl-2,4-nonanedione 1, 1-methyl-2-oxopropyl-hexanoate 2 and 1-methyl-2-oxopropyl-acetate 3 are volatile compounds contributing to the characteristic flavour of a green tea brew.1
Here we report the stereoselective chemo-enzymatic synthesis of compound 1 with acetylacetoin synthase (AAS)2 and its use to determine the stereochemistry of the natural product by chiral GC-MS analysis. Furthermore, on the basis of known rearrangements of α-hydroxy-β-diketones3, we propose an alternative mechanism for the formation of 3 and 4 based on thermal and/or basic degradation of 1.

Many volatile compounds identified in complex biological matrixes are chiral molecules. The chirality evaluation is of great importance in drugs and pheromone chemistry, but chirality also plays a very important role in flavour and fragrance chemistry. Analyses of these products are performed to determine their composition, to control their quality and to identify adulterations or product counterfeiting. In this respect, proper characterizations of the enantiomeric composition can provide information on the genuineness, geographic origin, biogenesis and quality of the product.
3-hydroxy-3-methyl-2,4-nonanedione 1, 1-methyl-2-oxopropyl-hexanoate 2 and 1-methyl-2-oxopropyl-acetate 3 are volatile compounds contributing to the characteristic flavour of a green tea brew.1
Here we report the stereoselective chemo-enzymatic synthesis of compound 1 with acetylacetoin synthase (AAS)2 and its use to determine the stereochemistry of the natural product by chiral GC-MS analysis. Furthermore, on the basis of known rearrangements of α-hydroxy-β-diketones3, we propose an alternative mechanism for the formation of 3 and 4 based on thermal and/or basic degradation of 1.